How big is your tank? Are there plenty of plants & hidey holes? If you really want to euthanize do it humanely. From what I've read the most humane way is to get a bowl of water, put it the freezer until its slushy, put the fish in it, then put it in the freezer. This IMO should be the absolute last resort.
I'm surprised your local pet stores won't take the fish. Our Petco even took in a couple giant paku and a huge knife fish and are adopting them out free because people moved and couldn't take their fish. (each fish is as big or bigger then a dinner plate) I know this was the store managers decision and would vary from store to store but it's just surprising. Go klonk your pet stores on the head. :D (kidding of coarse)